A growing fraction of plastic resins produced today are blends of two or more polymers. Polymer blending offers an extraordinary rich range of new materials with enhanced characteristics regarding mechanical, chemical or optical performances. However, most commercial blends are immiscible because nearly all polymer pairs cannot be soluble in each other. When two immiscible polymers are blended during melt extrusion, a stable morphology is reached in which one phase is mechanically dispersed inside the other. The size and shape of the dispersed phase depends on several processing parameters including rheological and interfacial properties, and the composition of the blend. By using the conventional mixing machines, the experimental limitation of domain size has been reported to be approximately 100 nm and 350 nm for Newtonian systems and polymer blend systems, respectively. Several methods of reducing phase size and improving interfacial adhesion for the practical application of polymer blend materials have been developed. Currently, a phase structure on the micrometer or submicrometer scale, that is, microstructured blends, is technically easy to prepare using typical processing methods, such as extrusion or injection molding. However, the preparation of nanostructured polymer blends for immiscible polymers, with a phase size of less than 100 nanometers, is very challenging using normal processing methods currently available. Very recently, nanostructured blends have been produced from block copolymers by using conventional melt processing, but the method shows obvious limitation for the practical application.